Such a method is already known in the art, e.g. from the recommendation entitled “Asymmetric Digital Subscriber Line (ADSL) Transceivers”, ref. G.992.1, published by the International Telecommunication Union (ITU) in June 1999.
FIG. 1 depicts two Digital Subscriber Line (DSL) transceiver units TU1 and TU2 connected to each other via a twisted pair of copper wires L. The transceiver unit TU1 is housed in a Digital Subscriber Line Access Multiplexer (DSLAM) at a central office CO, while the transceiver unit TU2 sites at customer premises CP.
A first communication path PATH1 is established over the line L from the transceiver unit TU1 to the transceiver unit TU2, and a second communication path PATH2 is established over the line L from the transceiver unit TU1 to the transceiver unit TU2.
The direction of communication from the central office CO to the customer premises CP is referred to as the downstream direction. The direction of communication from the customer premises CP to the central office CO is referred to as the upstream direction.
DSL transceiver units make use of Frequency Domain Division (FDD) to achieve duplex communication over a shared medium. Upstream and downstream directions are assigned disjoint frequency bands.
Frequency allocation for Very high speed Digital Subscriber Line (VDSL) is depicted in FIG. 2, where downstream and upstream directions are denoted as DS and US respectively. It is noteworthy that upstream direction is assigned higher frequency bands than downstream direction is.
Attenuation on most physical channels increases with both frequency and distance. It might be difficult to achieve a certain target data rate over long lines and/or at high frequencies.
The maximum data rate that can be achieved over a communication path is given by the well-known Shannon formula, and is a function of the Signal-to-Noise Ratio (SNR).
DSL transceiver units accommodate a Low Noise Amplifier (LNA) with Automatic Gain Control (AGC) that automatically tunes the analog gain in order to fit the received signal within the range of the Analog-to-Digital Converter (ADC), thereby minimizing the quantization noise engendered by the ADC.
DSL transceiver units further accommodate an hybrid that separates the transmit or near-end signal going to the line from the receive or far-end signal coming from the line. Unfortunately, the hybrid has no perfect echo rejection and an amount of the near-end signal mixes with the far end signal.
In VDSL, emphasis was put on flexibility for the allocation of the 138 kHz-12 MHz frequency band to upstream and downstream communication. The drawback of this flexible design is the inability to separate in the analog domain the local echo from the far-end signal. The echo will then contribute to the average receive power as measured for AGC.
The disclosed method is disadvantageous for long lines. The far end signal is very small and decreasing with line length, the quantization noise is constant because it is mostly determined by the echo. This means that the SNR, and thus the capacity, is decreasing until, at a certain line length, the requested target data rate can no longer be achieved.
The effect of attenuation will be different for upstream and downstream direction. The more bands of higher frequency are used for a certain communication direction, the more this direction will be affected by the attenuation and the higher the loss in capacity will be for this direction. As a consequence, the requested service will typically not be met in only one of the two directions.